1. Field of the Invention
The present invention relates to a pneumatic tire comprising a tread surface that has a plurality of lug grooves opening to sidewall thereof, particularly to a pneumatic tire intended for use for rough road running.
2. Description of the Related Art
Pneumatic tires intended for use for rough road running (so-called, off-road tires) are expected to run not only on flat paved roads and/or unpaved roads but also on rough roads including muddy grounds. Particularly, during driving on a muddy ground, pneumatic tires may be often caught by mud and steering to change a direction may often be lost due to wheel ruts made by the tires themselves. Under such conditions where the direction is hardly changed to a driver's intended direction, a vehicle may unintentionally proceed into an area with a worse condition of road surface state. In a worst case, the tires may be caught by the mud and the vehicle may get stuck, therefore the vehicle may not be able to proceed anymore. In order to prevent such stuck, the tires need to have traction performance not only on the tread surfaces of pneumatic tires but also on the sidewall.
Japanese Unexamined Utility Model Publication No. 60-118502 proposes a pneumatic tire having a plurality of lug grooves. The pneumatic tire is formed with a plurality of protruding strips disposed on the groove bottom of each of the lug grooves; to thereby prevent the lug grooves being filled with mud during running on muddy ground. In addition, Japanese Unexamined Patent Publication No. 10-230708 proposes a pneumatic tire having a plurality of lug grooves, and the bottom surface of which is covered with a mud prevention layer made of an elastic layer having a low hardness. Further, on the surface of the mud prevention layer, a plurality of substantially straight grooves are disposed with a predetermined distances in a longitudinal direction of the lug groove; to thereby prevent the lug grooves from being filled with mud during running on a muddy ground. However, any of the above pneumatic tires fails to provide satisfactory removing performance of mud caught in the lug grooves, and accordingly, the tread surface does not satisfactorily perform the traction performance. Additionally, in the above pneumatic tires, the sidewall does not provide sufficient traction performance, thereby resulting in a poor running performance on a rough road.
Japanese Unexamined Patent Publication No. 2008-49751 describes a pneumatic tire having a tire surface of a block pattern formed with a plurality of blocks. The pneumatic tire has a plurality of lateral grooves each opened to a ground-contact point in a width direction. The groove bottom of each of the lateral grooves has a protrusion extending along a direction of the lateral groove. As viewed from the front side of the tire, the protrusion has a start point thereof closer to the center than the ground-contact point, and an end point thereof is located at the outer side than the ground-contact point beyond the ground-contact point. In this pneumatic tire, the lug grooves are effectively prevented from being filled with mud; and thus, the traction performance by the tread surface is increased. However, in view of the traction performance by the sidewall, the pneumatic tire still has a room to be further improved in running performance on a rough road.
Japanese Unexamined Patent Publication No. 2004-291936 describes a pneumatic radial tire having a plurality of buttress portions on which a plurality of protrusions of 5 mm or more in height is disposed with a distance in a circumferential direction of the tire. The length of the surface of each of the protrusions in a circumferential direction and the length in a tire diameter direction are larger than the height of the protrusion, and the area increases from the surface to the bottom of the protrusion. The area of the bottom of the protrusion is twice as large as the area of the surface of the protrusion. This pneumatic tire is superior in the traction performance by the sidewall. However, in view of the removing performance of mud caught in the lug grooves, there still is a room for further improvement in running performance on a rough road.